1. Field of the Invention
The present invention relates to durable raised pavement markers (DRPM's), that are used for traffic markings and delineation. More particularly, the invention relates to DRPM's that are cast using a fiber-reinforced composite capable of providing a high apparent flexural modulus and impact strength to resist vehicle impact.
2. Related Art
Raised markers are used as delineators for traffic lanes to allow drivers of oncoming vehicles to correctly position themselves on the roadway, especially at night or under poor driving conditions. Roadway delineation is accomplished by retroreflective elements that are attached to the face of the raised marker. The retroreflective elements return light from vehicle head lights back to the driver.
Raised pavement markers have been commonly used for many years, and a most successful raised pavement marker is a potted shell type described in U.S. Pat. No. 3,332,327 to Heenan. The shell is typically formed from an acrylic resin and is potted with a filled epoxy resin. These markers tend to break up under repeated impact from vehicles and therefore are likely to require frequent replacement. Under high traffic conditions or when traffic excessively impacts on the markers, failure may occur in only a few months.
Attempts have been made to reinforce the marker shell and potting filler. For example, U.S. Pat. No. 5,002,424 to Hedgewick discloses placing extending ribs in the shell to add additional anchorage to the shell, and filling the shell with an epoxy resin potting material. U.S. Pat. No. 5,340,231 to Steere et al. also discloses a potted shell marker. Steere et al. teach the use of a shell made of a long-fiber reinforced thermoplastic material for high impact-resistance. The marker utilizes a hollow ribbed housing constructed for flexure and strength at elevated temperature. U.S. Pat. No. 5,403,115 to Flader suggests the use of a glass fiber reinforcement in the potting filler, sometimes in combination with a fiberglass mat as further reinforcement for the base. The application notes that adding about one to three percent by weight of chopped fiberglass in the fill results in optimum strength while greater than three percent presents processing problems. The above designs recognize the need for high impact resistance and high flexural modulus but attempt to achieve these properties using a potted shell.
U.S. Pat. No. 3,164,071 to Rubenstein discloses traffic markers having a core made from a rubber-concrete mixture. The core may be laminated with a resin-impregnated fiberglass mat. The core may also be infused with resin or a resin-fiber integument during the lamination process. The marker disclosed by Rubenstein is relatively difficult to make, and voids caused by incomplete infusion may lead to premature failure. Markers of the type taught by Rubenstein have not become commercially successful.
Some pavement markers have been made without an exterior shell. Porcelain clay markers, for example, have achieved commercial success. However, they suffer from shattering on repeated impact, especially on soft roads. In addition, a porcelain marker generally requires significant energy to create, and can present difficulties in permanently attaching a retroreflective element to its exterior.
Since the mid-1980's, the Traffic Control Materials Division of the assignee of the present application (Minnesota Mining and Manufacturing Company, hereafter "3M") has been designing and marketing raised pavement markers. These pavement markers have been made from an injection molded high impact-resistant engineering thermoplastic polycarbonate (PC). U.S. Pat. No. 4,875,798 to May describes markers of this type. The 3M DRPM body design has been generally rectangular in transverse cross-section, with a rounded top and sloping sides. The rounded top allows the impact forces to concentrate on the thickest part of the marker, while providing the added benefit of daytime visibility. The sloping sides provide stress relief from the high compressive impact force and also provide additional surface area for daytime visibility. The use of high impact-resistant engineering thermoplastic PC further increases daytime visibility. But more importantly, the PC material is selected for its high performance impact resistance. The benefit derived from this feature is reduced breakage and cracking in the marker body.